Thin-film resistor and method of fabrication

ABSTRACT

A thin-film resistor has: 1. a resistance layer positioned on a dielectric layer, 2. a protective layer positioned on the resistance layer and having two openings on two ends of the resistance layer, 3. an insulating layer covering the upper and side surfaces of the protective layer, the side surfaces of the resistance layer, and the surface of the dielectric layer, the protective layer having two openings above the two openings of the protective layer, 4. two plugs positioned in the two openings of the insulating layer and the protective layer for electrically connecting to the two ends of the resistance layer, and 5. two conductive layers formed on the insulting layer and positioned on the two plugs, and which are used as two electrical wires for electrically connecting to the two ends of the resistance layer.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a thin-film resistor, and moreparticularly, to a thin-film resistor for use on a semiconductor wafer.

[0003] 2. Description of the Prior Art

[0004] There are many kinds of resistive components in the ICs of asemiconductor wafer, such as the gate conductive layer of thesemiconductor wafer, doped layers as a resistance component, orthin-film resistors. The main problem with gate conductive layers anddoped layers is that the resistance of both is too low. Therefore, thesecomponents, if used, must be made large to increase their resistance tosufficient levels. It is clear that the gate conductive layers and thedoped layers are not suitable for use in semiconductor processes withsmall line-widths. Also, since gate conductive layers and doped layerscomprise silicon as a conducting material, the conductivity of theresistance component easily changes with temperature variations, makingthe resistance of these resistive components very unstable. If aresistive component with a low conductivity and a stable resistance isrequired for an IC, use of a thin-film resistor is essential.

[0005] Please refer to FIG. 1 and FIG. 2. FIG. 1 and FIG. 2 areschematic diagrams of the method of forming a thin-film resistor 18according to the prior art. A thin-film resistor 18 of the prior art isformed on the surface of the dielectric layer 10 of a semiconductorwafer 11. First, a resistance layer 12 and a protective layer 14 aresequentially formed within a predetermined area on the surface of thedielectric layer 10. Next, a conducting layer 16 made of an aluminumalloy is formed on the surface of the dielectric layer 10 and theprotective layer 14, as shown in FIG. 1. A wet etching process is thenperformed to remove all of the conducting layer 16 and the protectivelayer 14 on the resistance layer 12 except for at the two ends of theresistance layer 12. This remaining portion is used as electricalconnecting wires for the two ends of the resistance layer 12. FIG. 2illustrates the completed thin-film resistor 18.

[0006] The wet-etching process is an isotropic process, and so theamount of sideways etching is approximately equal to the amount ofvertical etching. Since the thin-film resistor 18 patterns theconducting layer 16 by wet-etching, it is essential that the resistancelayer 12 and the protective layer 14 have large surface areas so thatthe most of the conducting layer 16 and the protective layer 14 on thesurface of the resistance layer 12 can be removed. At the same time, theconducting layer 16 and the protective layer 14 at the two ends of theresistance layer 12 are maintained. Because of this, the prior artmethod of forming the thin-film resistor 18 can only be used inprocesses with a line-width of 3 μm or greater, and cannot be used inprocesses with smaller line widths.

SUMMARY OF THE INVENTION

[0007] It is therefore a primary objective of the present invention toprovide a thin-film resistor for use in a semiconductor wafer, and amethod of forming the same to solve the above-mentioned problems.

[0008] In a preferred embodiment, the present invention provides athin-film resistor on a dielectric layer of a semiconductor wafer. Aresistance layer is positioned in a predetermined area of the dielectriclayer. A protective layer is positioned on the resistance layer in thepredetermined area and has two openings on two ends of the resistancelayer. An insulating layer is formed on the semiconductor wafer andcovers the upper and side surfaces of the protective layer, the sidesurfaces of the resistance layer, and the surface of the dielectriclayer outside of the predetermined area. The protective layer has twoopenings above the two openings of the protective layer. Two plugs arepositioned in the two openings of the insulating layer and theprotective layer for electrically connecting to the two ends of theresistance layer. Finally, two conductive layers are formed on theinsulting layer and are positioned on the two plugs. The two conductivelayers are used as two electric wires for electrically connecting to thetwo ends of the resistance layer.

[0009] It is an advantage of the present invention that the thin-filmresistor of the present invention has a stable resistance and can beused in processes with smaller line-widths to reduce the overall area ofthe semiconductor product.

[0010] These and other objectives of the present invention will no doubtbecome obvious to those of ordinary skill in the art after having readthe following detailed description of the preferred embodiment, which isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 and FIG. 2 are schematic diagrams of the method of forminga thin-film resistor according to the prior art.

[0012]FIG. 3 is a cross-sectional diagram of a thin-film resistoraccording to the present invention.

[0013]FIG. 4 to FIG. 8 are schematic diagrams of the method of forming athin-film resistor according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION Structure

[0014] Please refer to FIG. 3. FIG. 3 is a cross-sectional diagram of anovel thin-film resistor 40 according to the present invention. As shownin FIG. 3, the thin-film resistor 40 is formed on a inter-layerdielectric (ILD) 20, and comprises a resistance layer 24 interposedbetween an isolating layer 22 and a protective layer 26, thereby forminga sandwiched resistor structure. The resistance layer 24 may be formedof metal or metallic compounds, such as chromium silicon, nickelchromium, or so forth. The stacked sandwiched resistor structure has arelatively small surface area, and is fabricated and defined byconventional photolithographic and etching techniques, which arewell-known methods in the art, to expose portions of the resistancelayer 24 and the isolating layer 22. Notably, the protective layer 26 ispositioned on the resistance layer 24 in the defined area, and comprisestwo wet-etched openings 28, which are formed using a dry-etchedinsulating layer 30 as a wet-etching mask. The wet-etched openings 28are positioned atop the two ends of the resistance layer 24. Theinsulating layer 30 is formed on the semiconductor wafer by aconventional CVD (chemical vapor deposition) process and covers theexposed surfaces of the protective layer 26, the resistance layer 24,and the dielectric layer 20. The protective layer 26 may be composed ofsilicon nitride, silicon oxy-nitride, or so forth. The isolating layer22 may be composed of silicon nitride or silicon dioxide.

[0015] In FIG. 3, the insulating layer 30 comprises two dry-etchedopenings 32 above the two wet-etched openings 28 in the protective layer26, which are simultaneously defined with the contact holes 50 using onephotomask. In the dry-etched openings 32, wet-etched openings 28 and thecontact holes 50, tungsten plugs 34, which have a patterned metal alloyadhesive layer 41 underneath the plugs, are formed to fill the openings32, 28 and the contact holes 50 by way of a conventional metaldeposition process and an etch back process. The tungsten plugs 34 areused to electrically connect to the two ends of the resistance layer.Patterned conductive layers 36, which are also used as two electricalwires for electrically connecting to the two ends of the resistancelayer, are formed on the insulting layer and are positioned on theplugs. The conductive layer 36 may be formed of aluminum, copper, or analuminum-copper alloy.

Process of the Preferred Embodiment

[0016] Please refer to FIG. 4 to FIG. 8. FIG. 4 to FIG. 8 arecross-sectional diagrams illustrating the method of forming a thin-filmresistor 40 according to the present invention. As noted, the thin-filmresistor 40 of the present invention is formed on a dielectric layer 20of a semiconductor wafer 21. The dielectric layer 20 may be formed ofborophosphosilicate glass (BPSG), phosphosilicate glass (PSG), SiO₂, orso forth. First, as shown in FIG. 4, an isolating layer 22 of siliconnitride and a resistance layer 24 of chromium silicon are sequentiallydeposited on the surface of the dielectric layer 20. A protective layer26 of silicon nitride is next formed on the resistance layer 24. Alithographic process and an anisotropic plasma dry-etching process areused to define an island consisting of a sandwiched stacked structure onthe dielectric layer 20, and thus expose portions of the resistancelayer 24 and the isolating layer 22. An insulating layer 30 of siliconoxide is then formed over the semiconductor wafer 21 by a CVD process tocover the exposed surfaces of the protective layer 26 and the resistancelayer 24 of the island, as well as the surface of the dielectric layer20 outside of the island.

[0017] As shown in FIG. 5, a second lithographic process and a seconddry-etching process are performed on the insulating layer 30 to form twodry-etched openings 32 extending down to the surface of the protectivelayer 26. In the second lithographic process and dry-etching process,two contact holes 50 are formed in the insulating layer 30 and thedielectric layer 20 using the same photomask by which the openings 32are defined. The contact holes 50 are used as a path for electricallyconnecting to other components on the semiconductor wafer 21.

[0018] As shown in FIG. 6, subsequently, a wet-etching process withphosphoric acid (H₃PO₄) that does not affect the insulating layer 30,the dielectric layer 20 and, most importantly, the resistance layer 24,is then performed on the protective layer 26 through the two openings 32of the insulating layer 30 to form two isotropically wet-etched openings28 extending down to the resistance layer 24.

[0019] Next, as shown in FIG. 7, an adhesive layer 41 and a tungstenlayer 34 are sequentially formed on the surface of the semiconductorwafer 21, the surface of the two openings 28 inside the insulating layer30 and the protective layer 26, and on the surface of the contact holes50. The adhesive layer 41 comprises a titanium layer, and a titaniumnitride layer underlying the titanium layer. The titanium nitride layeris used as a stumbling layer for isolating the tungsten layer 34 and thetitanium layer. The tungsten layer 34 inside the openings 28 serves asplugs 34, and the tungsten layer 34 inside the contact holes 50 servesas plugs 35. Then, an etch back process is performed on the surface ofthe semiconductor wafer 21 to remove the tungsten layer 34 from theinsulating layer 30 so that the top end of each of the plugs 34, 35 isat approximately the same height as the surface of the insulating layer30.

[0020] Finally, as shown in FIG. 8, a conducting layer 36 made of analloy consisting mostly of aluminum is deposited on the surface of thesemiconductor wafer 21. A lithographic process and a metallic etchingprocess is then performed to remove the conducting layer 36 and theadhesive layer 41 outside a preselected area so as to form a pluralityof conducting layers 36 on the surface of each of the plugs 34, 35. Thiscompletes the thin-film resistor 40 of the present invention. Becausethe plugs 34 in the two openings 28 can electrically connect to the twoends of the resistance layer 24, the plugs 34 and the two conductinglayers 36 above the plugs 34 can be used as electric wires for the twoends of the resistance layer 24. The plugs 35 in the contact holes 50,and the conducting layers 36 above the plugs 35, can be used as electricwires to electrically connect to the other components on thesemiconductor wafer 21.

[0021] As shown in FIG. 8, the thin-film resistor 40 of the presentinvention comprises the resistance layer 24 positioned within apredetermined area on the surface of the dielectric layer 20, theprotective layer 26 with openings 28 positioned in the predeterminedarea on the resistance layer 24; the insulating layer 30 covering thesurface and the sides of the protective layer 26, the sides of theresistance layer 24 and the surface of the dielectric layer 20 outsidethe predetermined area; two plugs 34 installed separately in the twoopenings 28 of the insulating layer 30 and the protective layer 26 andalso connecting to the two ends of the resistance layer 24, and twoconducting layers 36 installed on the two plugs 34 so that the twoconducting layers 36 and the plugs 34 can be used as electrical wires toconnect to the resistance layer 24.

[0022] In the thin-film resistor 40 of the present invention, theisolating layer 22 below the resistance layer 24 isolates out-gassinggenerated from the BPSG of the dielectric layer 20 to prevent theout-gassing from affecting the resistance value of the resistance layer24. The protective layer 26 protects the underlying resistance layer 24from plasma damage caused by subsequent dry-etching processes. Also, thetwo openings 28 of the protecting layer 26 are formed by wet-etching anddo not affect the resistance layer 24. Consequently, the resultingresistance of the resistance layer 24 of the thin-film resistor 40 ofthe present invention displays superior stability over widely varyingtemperatures.

[0023] In the thin-film resistor 40 of the present invention, the sidesurfaces of the resistance layer 24 are covered by the insulating layer30. Therefore, the metallic conducting layer 36 is able to connect toother components of the semiconductor wafer 21 without contacting theside of the resistance layer 24. This prevents short-circuiting. As aresult, there are fewer restrictions on the design of the metallicconducting layer 36. Also, other than the two openings of the protectivelayer 26 being made by a wet-etching process, all other etchingprocesses are anisotropic dry-etching processes. Therefore, the area ofthe resistance layer 24 can be very small, with only the plugs 34 andthe overlying conducting layers 36 serving as electrical connectingwires of the resistance layer 24. The present invention is suitable forprocesses with line-widths below 0.5 μm.

[0024] Compared to the thin-film resistor 18 of the prior art, in thepresent invention the thin-film resistor 40 and the method for itsformation, the resistance layer 24 is sandwiched between an overlyingprotective layer 26 and the underlying isolating layer 22. Theinsulating layer 30 is then deposited onto the surface of thesemiconductor wafer 21, thus stabilizing the resistance of theresistance layer 24. Also, the openings 28 in the protective layer 26are formed by wet-etching, but all other etching processes areanisotropic dry-etching processes. Therefore, the area of the resistancelayer 24 can be as small as possible. The present invention method notonly produces a stable resistance thin-film resistor 40, but also may beused in processing with line-widths below 0.5 μm to reduce the area ofthe semiconductor product.

[0025] Those skilled in the art will readily observe that numerousmodifications and alterations of the device may be made while retainingthe teaching of the invention. Accordingly, the above disclosure shouldbe construed as limited only by the metes and bounds of the appendedclaims.

What is claimed is:
 1. A thin-film resistor positioned on a dielectric layer of a semiconductor wafer, the thin-film resistor comprising: a resistance layer positioned in a predetermined area of the dielectric layer; a protective layer positioned on the resistance layer in the predetermined area and comprising two openings on two ends, respectively, of the resistance layer; an insulating layer formed on the semiconductor wafer and covering the upper and side surfaces of the protective layer, the side surfaces of the resistance layer, and the surface of the dielectric layer outside the predetermined area, the insulating layer comprising two openings respectively above the two openings of the protective layer; two plugs respectively positioned in the two openings of the insulating layer and the protective layer for electrically connecting to the two respective ends of the resistance layer; and two conductive layers formed on the insulting layer and respectively positioned on the two plugs, the two conductive layers being used as two electrical wires for electrically connecting to the two respective ends of the resistance layer.
 2. The thin-film resistor of claim 1 wherein the resistance layer is formed of CrSi (chromium silicon), the protective layer is formed of silicon nitride by using a chemical vapor deposition method, the insulating layer is formed of silicon oxide by using a chemical vapor deposition method, and the dielectric layer is formed of borophosphosilicate glass (BPSG).
 3. The thin-film resistor of claim 2 further comprising an isolating layer positioned in the predetermined area and between the resistance layer and the dielectric layer, the isolating layer isolating out-gassing produced by the borophosphosilicate glass of the dielectric layer to prevent the out-gassing from affecting the resistance of the resistance layer.
 4. The thin-film resistor of claim 3 wherein the isolating layer is formed of silicon nitride or silicon oxide.
 5. The thin-film resistor of claim 1 wherein the plug is formed from a tungsten layer.
 6. The thin-film resistor of claim 5 further comprising the following two layers between each plug and each opening of the insulating layer and the protective layer: a titanium layer positioned on the surface of each opening of the insulating layer and protective layer which is used as an adhesive layer; and a titanium nitride layer positioned on the surface of the titanium layer which is used as a stumbling layer; wherein when forming the tungsten layer, the titanium nitride layer is used to isolate the tungsten layer and the titanium layer.
 7. The thin-film resistor of claim 1 wherein the two conductive layers are formed of a metallic alloy based on aluminum (Al). 